Add attributes to `std.socket`

`std.socket` now has `@safe` public API.

Also added few fixes.
> it sets us on a path of ABI breakage due to different mangling.

This is not an issue, we don't maintain ABI compatibility.

There is no phobos policy requiring or encouraging in on value arguments that I'm aware of, and it honestly seems like a completely useless thing to do in most cases.  Some people have advocated that in the past, but that doesn't make it more than some people's style preference.